I’ve never heard any discussion of this subject, or seen any test results, but I suspect that whether an email subject line is upper and lower case may have an influence on results.
Think about it: for most personal emails, lower case is the convention. Commercial emails tend to be upper and lower case, making them look more like a headline. It’s like announcing, “This is an ad.”
It wouldn’t surprise me if an initial cap on only the first word of the subject line would outpull the more common upper and lower case treatment. Anyone care to test and share results?